Interactions between Edge Localized Modes (ELMs) and the Scrape-Off Layer (SOL) plasma in EAST and HL-2A Tokamaks and a Plan for the External Control

Abstract

In the divertor tokamaks EAST and HL-2A, interactions between edge localized modes (ELMs) and the scrape-off layer (SOL) plasma were studied to clarify the existence of ELM-induced filamentary currents driven along the magnetic field line in the SOL. In both tokamaks, small toroidal plasma current spikes of the order of a few percentages of the total plasma current have been observed in H-mode plasmas with large amplitude ELMs. The characteristics of the current spikes were studied in various divertor configurations by changing the direction of the toroidal drift with the fixed direction of the plasma current. The observed current spikes have a similar waveform with those of Da emission light in the divertor region and the ion saturation currents on the divertor targets. In all shots analyzed, the current spikes are positive to the total plasma current. Sharp drop in the loop voltage immediately induced as an action of the poloidal magnetic flux conservation. The origin of the observed plasma current spikes is discussed, based on thermoelectric current and other SOL current driving mechanisms. A possibility of active control of SOL currents by electric biasing of toroidally separated two divertor targets on HL-2A is also discussed.